Electrical isolation barriers can be identified in many industrial, medical and communication applications where it is necessary to electrically isolate one section of electronic circuitry from another electronic section. In this context, isolation exists between two sections of electronic circuitry if a large magnitude voltage source (typically on the order of 1000 volts or more) connected between any two circuit nodes separated by the barrier causes less than a minimal amount of current flow (typically on the order of 10 milliamperes or less) through the voltage source. An electrical isolation barrier must exist, for example, in communication circuitry that connects directly to the standard two-wire public switched telephone network and that is powered through a standard residential wall outlet.
The Federal Communications Commission (FCC) has determined that residential telecommunications equipment (e.g., solid-state modems) should have surge protection up to a threshold voltage level (e.g., 1500 volts). In particular, the FCC regulations, Part 68, which governs electrical connections to the telephone network in order to prevent network harm, provides that an isolation barrier capable of withstanding 1000 volts rms (root mean square) at 60 Hz with no more than 10 milliamps current flow, must exist between circuitry directly connected to the two wire telephone network and circuitry directly connected to the residential wall outlet.
In order to achieve regulatory compliance, a conventional approach to electrical isolation is to ensure that there is enough spacing between the telephone network circuits and other circuits referenced to protective earth ground. The components permitted for use across this isolation barrier are limited to transformers, high voltage capacitors, optoisolators, relays and large resistors. However, it has been determined that in actual field usage, higher surge voltages (e.g. 3,000 to 5,000 volts) may occur across this isolation barrier. When such voltages are present, air may begin to ionize anywhere within the enclosed system; dielectric breakdown may occur within the isolation barrier components, including high voltage capacitors. In short, the damage to the system is undeterministic and may require the replacement of the entire system.
Direct Access Arrangement (DAA) circuitry may be used to terminate the telephone connections at a phone line user's end to provide a communication path for signals to and from the phone lines. DAA circuitry includes the necessary circuitry to terminate the telephone connections at the user's end and may include, for example, an isolation barrier, DC termination circuitry, AC termination circuitry, ring detection circuitry, and processing circuitry that provides a communication path for signals to and from the phone lines. It is also desirable that the DAA circuitry act as an isolation barrier to meet the requirements of FCC regulations, Part 68. Examples of DAA circuitry known in the art may be found described in U.S. Pat. No. 6,385,235 and in U.S. patent application Ser. No. 09/347,688 filed Jan. 2, 1999 and entitled “DIGITAL ACCESS ARRANGEMENT CIRCUITRY AND METHOD HAVING A SYNTHESIZED RINGER IMPEDANCE FOR CONNECTING TO PHONE LINES” by Tuttle et al., the disclosure of each being incorporated herein by reference.
In telecommunications equipment that utilize DAA circuitry, such as for example modem circuits, a high voltage surge can come from one of two sources: the telephone line or the power line. One of the most common causes of high voltage surges is a lightning strike, although telephone central office equipment and power equipment failures can also cause similar surges. Once a surge is introduced into a modem circuit, it can be classified into one of two types: metallic (differential) or longitudinal (common mode). Longitudinal surges occur between the wires in the transmission system and the system ground (often the physical earth). For a typical consumer phone line system the two wires are labeled tip and ring. Most surges caused by lightning striking the ground are longitudinal in nature. These surges cause tip and ring to rise, either positively or negatively, in voltage relative to earth. This stresses electronic components which bridge the isolation barrier, such as transformers, relays, opto-isolators, and capacitors. A metallic surge, which occurs between the two wires of a transmission system, is usually the result of a longitudinal event. In a typical telephone loop, the impedance from tip to earth is usually not identical to the impedance from ring to earth. This difference in impedances during a longitudinal surge yields a voltage from tip to ring. This stresses electronic components that connect across tip and ring
High voltage surges may also occur as multiple surge events, often caused by multiple lightning strikes. Contrary to popular belief that a lightning strike is a single bolt of lightning, a lightning strike is actually composed of a series of multiple strokes. The average number of strokes within a single lightning strike has been estimated to be about four. The duration of each lightning stroke varies, but has been estimated to average about 30 microseconds. The average peak power per stroke has been estimated to be about 1012 watts. Because these strokes are spaced close together in time (the median time between strokes has been estimated to be 60 ms), they can have what has been termed a “pump-up” effect on capacitive portions of a modem circuit. In this regard, a lightning strike can occur that has individual strokes with insufficient power to damage any components on a modem board. However, if a capacitive node is exposed to the surge, that node will temporarily store energy from each individual stroke. The amount of time it takes for this charge to dissipate is determined by the time constant of the specific capacitive circuit. If the lightning strike consists of multiple strokes, and if the multiple strokes are spaced close enough together such that charge placed on the capacitive node has insufficient time to dissipate, each stroke will result in energy being stored in the capacitor. If the cumulative amount of energy stored produces a voltage across the capacitor that causes the capacitor to break down, then damage to some part of the modem circuit (either the capacitor itself or the components nearby) can occur. This “pump-up” effect of capacitive nodes is challenging to reproduce in laboratory situations, but has been shown through laboratory simulations and experiments, as well as field study, to be a factor in DAA and modem damage.